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Jun 8, 2015 - Functionalized Poly(glycidyl methacrylate) Microspheres for. Catalytic Reduction of 4‑Nitrophenol. Wenchao Zhang, Yan Sun, and Lin Zha...
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In Situ Synthesis of Monodisperse Silver Nanoparticles on SulfhydrylFunctionalized Poly(glycidyl methacrylate) Microspheres for Catalytic Reduction of 4‑Nitrophenol Wenchao Zhang, Yan Sun, and Lin Zhang* Department of Biochemical Engineering and Key Laboratory of Systems Bioengineering of the Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China S Supporting Information *

ABSTRACT: Immobilization of silver nanoparticles (Ag NPs) to improve monodispersity and recyclability is crucial for applications in nanocatalysts. Herein, a novel protocol for stabilizer-free, effective, and in situ synthesis of Ag NPs on sulfhydrylfunctionalized poly(glycidyl methacrylate) microspheres (PGMA-SH) was proposed. Ag NPs of 16.97 ± 3.15 nm were successfully grown on PGMA-SH, and remained monodisperse and stable even after sonication, washing, and long-term storage. Moreover, the Ag NPs on PGMA-SH (Ag NPs@PGMA-SH) composite exhibited excellent catalytic activity with an average normalized activity parameter of 4.38 × 10−3 L·mg−1·s−1 toward the reduction of 4-nitrophenol, which was 1.3−132 times higher than reported in literature. The composite can be easily recycled and showed excellent reusability as a conversion higher than 92% was achieved after 10 cycles. Thus, the preparation of Ag NPs@PGMA-SH has been proven a feasible, straightforward, and effective protocol, which would facilitate the applications of Ag NPs in environmental control. polymers also have been used to stabilize other NPs.37,38 The size and distribution of the resulting NPs can be strongly affected by the surface density of sulfhydryl groups on sulfhydryl-containing polymers, which served as nucleation sites for the formation of NPs.39 Therefore, preparing sulfhydryl-functionalized PGMA (PGMA-SH) supports to realize the in situ synthesis of Ag NPs by Ag−S chemistry would be a promising strategy. In the present study, cystamine dihydrochloride was first used to introduce sulfhydryl groups on the surface of PGMA. The obtained PGMA-SH microspheres were then used as the support for the in situ fabrication of Ag NPs. The obtained Ag NPs on PGMA-SH (Ag NPs@PGMA-SH) composite was characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD) analysis, and ultraviolet−visible (UV−vis) spectroscopy. Thereafter, the catalytic activity and reusability of the as-prepared composite for the reduction of 4NP were evaluated.

1. INTRODUCTION Silver nanoparticles (Ag NPs) have attracted intensive attention because of their excellent optical, catalytic, and electrochemical properties.1−5 However, their applications usually suffer from irreversible aggregation and difficult recovery due to the high surface energy and large surface area.3,6,7 Immobilization8 of Ag NPs on a variety of supports including polymers,9−13 metal oxides,14,15 and carbon materials16,17 has been proven capable of addressing these issues, and then broadening the applications18 in, for example, photovoltaic cells,19 memory devices,20 and supercapacitors.21 Moreover, immobilizing nanoparticles on the supports22−25 can facilitate their reuse as a result of easy separation from the reaction mixture, and also make them free of stabilizer. However, weak interactions between the supports and Ag NPs give rise to the leakage, aggregation,26 or a poor distribution27 of NPs. In this regard, designing functional supports and preparing Ag composites with good stability are crucial. Poly(glycidyl methacrylate) (PGMA) microsphere, a polymer advantageous due to easy preparation, active surface for stable immobilization, and easy separation by simple centrifugation after reaction, has been extensively used as a support for noble metals.10,28−31 Nam et al.28 used aminefunctionalized PGMA microsphere as the template to fabricate gold crystals with high catalytic activity in converting 4nitrophenol (4-NP) to 4-aminophenol (4-AP). Li et al.32 deposited gold nanoparticles (12 ± 3 nm) on the surface of poly(allylamine hydrochloride)-modified PGMA spheres. Chen et al.10 prepared Ag NPs coated on PGMA colloidal spheres (Ag NPs@PGMA) with size-dependent antibacterial property. However, preparation of stable and uniform Ag NPs on PGMA microspheres without leakage in reaction is still a challenge. It is well-known that Ag tends to form a stable coordinate covalent bond with the sulfhydryl group.33−36 Sulfhydryl-containing © 2015 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. All reagents were of analytical grade and used without further purification. 4-Nitrophenol (4-NP) and sodium borohydride (NaBH4) were purchased from Guangfu Fine Chemical Research Institute (Tianjin, China). Cystamine dihydrochloride, glycidyl methacrylate (GMA), azobis(isobutyronitrile) (AIBN), and ethylene glycol dimethacrylate Received: Revised: Accepted: Published: 6480

March 17, 2015 June 3, 2015 June 8, 2015 June 8, 2015 DOI: 10.1021/acs.iecr.5b01010 Ind. Eng. Chem. Res. 2015, 54, 6480−6488

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Industrial & Engineering Chemistry Research

Scheme 1. Schematic Illustration (a) and Reaction Schematic (b) of in Situ Generation of Ag NPs on PGMA-SH Microspheres

(EDMA) were purchased from Alfa Aesar (Lancashire, U.K.). Silver nitrate (AgNO3) was from Tianjin Yingda Rare Chemical Reagents Factory (Tianjin, China). 5,5′-Dithiobis(2-nitrobenzoic acid) (DTNB), polyvinylpyrrolidone (PVP) K-30, DLdithiothreitol (DTT), and other reagents were purchased from Dingguo Biotechnology Co. (Beijing, China). 2.2. Synthesis of PGMA Microspheres. PGMA microspheres were synthesized using a dispersion polymerization method40 with minor modification. An 8 g sample of GMA, 2.4 g of PVP K-30, and 0.16 g of AIBN were dissolved in 72 g of methanol in a 250 mL three-necked flask using mild sonication, and then the solution was purged with nitrogen by stirring vigorously at 300 rpm for 30 min at room temperature (RT, 25 °C). Thereafter, the mixture was heated to 75 °C and refluxed for 2 h with stirring. Subsequently, 160 μL of EDMA was added to the solution, and the flow of nitrogen was then decreased through a nitrogen tank pressure regulator to minimize the stripping of monomer from the reaction mixture. The reaction was conducted at 75 °C and refluxed for 12 h. Then the product was allowed to cool to RT and washed several times with ethanol and deionized water. Finally, the PGMA microspheres were stored in 20% (v/v) ethanol at 4 °C. 2.3. Synthesis of Sulfhydryl-Functionalized PGMA Microspheres. A 1.5 g sample of PGMA microspheres was immersed in a solution of Na2CO3 (30 mL, 0.5 M), followed by the addition of 230.5 mg of cystamine dihydrochloride. The obtained mixture was then incubated at 45 °C with moderate shaking at 180 rpm for 12 h. Thereafter, the mixture was centrifuged and the precipitate (microspheres) was washed several times with deionized water. Then 200 mg of the aforesaid microspheres was resuspended in 10 mL of 12.4 mg/ mL DTT solution. The pH of the mixture was kept within a range of 7.5−8, and then the mixture was placed at RT for 2 h. Finally, the mixture was centrifuged and the precipitate was washed with ethanol and deionized water to remove the excess

DTT until the solution became neutral. The obtained microspheres were denoted as PGMA-SH. 2.4. Synthesis of Ag NPs Supported by PGMA-SH Microspheres. A certain volume (2, 3, or 4 mL) of 2.5 mM AgNO3 was introduced into 16 mL of 0.625 mg/mL PGMASH dispersion solution and then stirred for 30 min. Subsequently, 1 mL of 0.6 M freshly prepared ice-cold NaBH4 solution was added dropwise slowly to the reaction mixture with gentle stirring and the reaction was conducted for 3 h at RT. After the reaction, the product was stored at 4 °C or freeze-dried under vacuum at −40 °C for structure characterization. The obtained composite was denoted as Ag NPs@ PGMA-SH. 2.5. Characterization. The FTIR spectra were recorded with a Bruker Tensor 27 spectrometer (Bruker, Germany). The average particle size and the polydispersity index (PDI) of the PGMA microspheres were determined by Malvern Mastersizer 2000 (Malvern Instruments Ltd., U.K.). Zeta (ζ) potential measurements were conducted on a Zetasizer Nano ZS (Malvern Instrument Ltd., U.K.) at RT. The surface morphologies of the PGMA-SH microspheres and the Ag NPs@PGMA-SH composite were obtained by SEM using an S4800 instrument (Hitachi Co., Japan) on an accelerating voltage of 3 kV. TEM analysis was performed on a JEM-2100F electron microscope (JEOL Ltd., Japan) coupled with EDS. The samples for TEM were obtained by dispersing a small drop of the suspension onto a copper grid precoated with amorphous carbon. The histogram, mean size, and standard deviation were obtained by analyzing 300 Ag NPs in highly magnified TEM (HRTEM) images using the image analysis program Nano Measurer 1.2. The crystal structures of the synthesized samples were characterized by a D8 Focus X-ray diffractometer (Bruker, Germany) in the diffraction angle range 2θ = 30−90° at ambient temperature, using Cu Kα radiation at 40 kV and 40 mA. In addition, TEM image of microtomed Ag NPs@PGMA-SH composite was obtained on a 70 nm thick 6481

DOI: 10.1021/acs.iecr.5b01010 Ind. Eng. Chem. Res. 2015, 54, 6480−6488

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Industrial & Engineering Chemistry Research section prepared by a diamond knife. UV−vis absorption spectra were measured on a Lambda 35 UV−vis spectrophotometer (PerkinElmer, USA). 2.6. Catalytic Reduction of 4-NP. To evaluate the catalytic activity of the developed Ag NPs@PGMA-SH composite, the reduction of 4-NP by NaBH4 was carried out in batch experiments as follows. A 3 mL volume of 1 mM 4-NP was mixed with fresh NaBH4 solution (9 M, 1 mL). Deionized water was then added to adjust the volume to 44 mL. Subsequently, 1 mL of aqueous dispersion of 0.5 mg of Ag NPs@PGMA-SH composite was rapidly added to the above reaction mixture to start the reduction. Immediately after that, 3 mL of suspension at a given interval was sampled and filtered through 0.22 μm membrane filters, and the UV−vis absorption spectra were then recorded at RT in a scanning range of 200− 500 nm. In all runs to be discussed here, the concentration of NaBH4 was chosen to largely exceed the concentration of 4NP. Thus, the kinetics of the reduction can be treated as pseudo-first-order in 4-NP concentration, which simplifies the present analysis. The pseudo-first-order rate constant of the reaction catalyzed by the Ag NPs@PGMA-SH composite catalyst was calculated by measuring the disappearance of the characteristic peak, viz., the concentration of 4-NP quantitatively at 400 nm collected at given intervals.41 The rate constant can be calculated using the formula

Figure 1. FTIR spectra of PGMA microspheres (a), PGMA-SH microspheres (b), and Ag NPs@PGMA-SH composite (c).

PGMA-SH (Figure 1b), indicating the decrease of epoxy group. Moreover, in the spectrum of PGMA-SH, two new peaks appeared at 1409 and 3365 cm−1 which could be attributed to vibrations of −NH. A weak peak appeared at 2560 cm−1, indicating the stretching of −SH and the existence of sulfhydryl groups. These results demonstrated the successful modification of PGMA surface and synthesis of PGMA-SH microspheres (Scheme 1). The average particle size and ζ potential of PGMA-SH were then determined to examine the modification process (Table 1). No significant change was observed on the particle size after

ln(A t /A 0) = −kappt

Table 1. Particle Sizes and ζ Potentials of PGMA, PGMASH, and Ag NPs@PGMA-SH

where kapp is the apparent rate constant, t is the reaction time, A0 is the initial concentration of 4-NP, and At is the concentration of 4-NP at time t. Because the kapp was dependent on the amount of Ag NPs used in the reaction, an “activity parameter” k was calculated by normalizing the k app value with respect to the mass concentration of catalyst.42 The amount of silver was determined by the mass balance of Ag in the reaction. Then the activity parameter was calculated and used to evaluate the catalytic activity of the as-prepared Ag NPs@PGMA-SH.

PGMA PGMA-SH Ag NPs@PGMA-SH

diameter (μm)

ζ potential (mV)

1.06 ± 0.09 1.12 ± 0.10 1.10 ± 0.08

2.7 ± 0.2 22.8 ± 1.2 17.8 ± 2.3

the modification, where the particle size of PGMA or PGMASH was ca. 1.10 μm. However, obvious increase of the ζ potential was observed from +2.7 mV of PGMA to +22.8 mV of PGMA-SH, as a result of the imino groups obtained in the modification43 (see Scheme 1b). The increased ζ potential confirmed the successful introduction of cystamine onto the PGMA microspheres, which was further confirmed by the presence of free sulfhydryl groups as revealed by a yelloworange color in a positive Ellman test44 (Supporting Information, Figure S1). According to the SEM (Figure 2a) and TEM images (Figure 2b), the obtained PGMA-SH microspheres were proven to be monodisperse, spherical with a nonporous, smooth surface. The particle size determined in these images was 1.02 μm, which was consistent with the size determined by dynamic light scattering (DLS; Table 1). The results indicated that the modification process did not have significant influence on the particle size and morphology of PGMA microspheres, because the sulfhydrylation reaction occurred homogeneously at the surface. The obtained uniform surface/interfacial microstructures of PGMA-SH microspheres make it an effective support for Ag NPs. 3.2. Synthesis of Ag NPs@PGMA-SH Composite. Synthesis of Ag NPs@PGMA-SH composite was then performed, and the effect of the amount of AgNO3 was examined. As illustrated in Scheme 1, the Ag+ can be accumulated on the surface of PGMA-SH through the strong

3. RESULTS AND DISCUSSION 3.1. Synthesis of PGMA-SH Microspheres. The in situ fabrication of Ag NPs@PGMA-SH composite is illustrated in Scheme 1. PGMA surface was treated with cystamine dihydrochloride through the covalent reaction between the epoxy groups of PGMA and the amino groups of cystamine dihydrochloride. The symmetrical and simple structure of cystamine dihydrochloride without other reactive groups (Scheme 1b) is helpful to achieve high reaction efficiency and to reduce side reactions or cross-linking. Even if both two amino groups of a cystamine dihydrochloride form covalent bonds with PGMA (cause cross-linking), it will be disrupted in the following step where DTT is used to disrupt the disulfide bonds of cystamine dihydrochloride to obtain a sulfhydrylterminated surface (PGMA-SH). The FTIR spectrum of PGMA-SH was then obtained to characterize the change of surface, as shown in Figure 1, where the spectrum of PGMA was used as control. For PGMA (Figure 1a), an obvious peak at 1730 cm−1 is shown, corresponding to the characteristic peak of the carbonyl group (see Scheme 1b) and confirming the successful fabrication of PGMA. Two peaks, one at 848 cm−1 and the other at 908 cm−1, were observed, corresponding to the epoxy group. These two peaks were weakened in the spectrum of 6482

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Figure 2. SEM (a) and TEM images (b) of PGMA-SH microspheres. Each inset shows an enlarged view.

interaction with −SH, and then reduced to realize the in situ formation of Ag NPs on PGMA-SH microspheres. After the addition of 3 mL of AgNO3 into the reaction solution, a color change from white for PGMA-SH to light yellow was observed (Supporting Information, Figure S2), demonstrating the formation of Ag NPs on the surface of microspheres, which was further confirmed by the FTIR spectrum (Figure 1c) and ζ potential (Table 1) of Ag NPs@PGMA-SH composite. As compared with the FTIR spectrum for PGMA-SH, the band belonging to −SH disappeared, indicating the formation of Ag−S interaction. A slight decrease of the ζ potential to +17.8 mV was observed in Ag NPs@PGMA-SH due to the coverage of imino groups, indicating the successful synthesis of Ag NPs on PGMA-SH microspheres. Moreover, no Ag NPs were examined in the supernatant after the loading of Ag NPs, as revealed in the UV−vis spectra of supernatant (Supporting Information, Figure S3), indicating that the Ag NPs were only formed on PGMA-SH.45 Then the amount of Ag NPs obtained was determined by the mass balance of Ag. However, with more AgNO3 added in the reaction solution (e.g., 4 mL), free Ag NPs appeared and serious aggregation of Ag NPs was observed on PGMA-SH supports (Supporting Information, Figure S4). Therefore, the appropriate amount of AgNO3 was designated as 3 mL herein. The size and morphology of synthesized Ag NPs@PGMASH composite were then determined using the typical SEM and TEM images at different magnifications (Figure 3). As compared with the smooth surface of uncoated microspheres (Figure 2), small dots were observed on the surface of Ag NPs@PGMA-SH composite, indicating stable attachment of Ag NPs on the surface. Moreover, it can be seen from the TEM image of microtomed Ag NPs@PGMA-SH composite (Supporting Information, Figure S5) that Ag NPs were grown predominantly on the surface of rather than inside PGMA-SH microspheres. Therefore, based on these SEM and TEM images, it can be confirmed that relatively monodisperse Ag NPs were uniformly distributed on the entire surface of microspheres, although agglomerates were inevitable, similar to what was reported elsewhere.17,46 The particle size of Ag NPs was measured as 16.97 ± 3.15 nm from the TEM image (Supporting Information, Figure S6). Furthermore, the crystalline structure of Ag NPs was determined in the highresolution TEM (HRTEM) image (Figure 4a). The interplanar spacing for the lattice planes was measured as 0.24 nm, which was consistent with the lattice spacing of Ag(111).47 The EDS spectrum for the obtained composite further confirmed the

Figure 3. SEM images of Ag NPs@PGMA-SH composite at different magnifications (a, 10 000; b, 60 000; c, 100 000 times) and TEM images of Ag NPs@PGMA-SH composite at different magnifications (d, 10 000; e, 25 000; f, 50 000 times).

existence of Ag and S elements in the hybrid composite (Figure 4b), indicating that the −SH groups were successfully introduced onto the PGMA microspheres and Ag NPs were successfully loaded. Moreover, C, N, and O elements from PGMA microspheres were confirmed in the EDS spectrum, with additional peaks of Cu attributed to the copper grid used. 6483

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Figure 4. HRTEM image (a) and EDS spectrum (b) of the Ag NPs@PGMA-SH composite.

generated in the crystalline state and well loaded on the surface of PGMA-SH microspheres. 3.3. Catalytic Activity of Ag NPs@PGMA-SH Composite. The catalytic activity of Ag NPs@PGMA-SH composite was then evaluated in the reduction of 4-NP to 4-AP by NaBH441 (schematically presented in Figure 6a). Herein, a shift of the absorbance peak from 317 to 400 nm was observed upon the addition of NaBH4 due to the formation of 4-nitrophenolate anions under alkaline condition49 (Supporting Information, Figure S7a). Although this reaction was a thermodynamically feasible process involving E0 for 4-NP/4AP = −0.76 V and H3BO3/BH4− = −1.33 V versus normal hydrogen electrode (NHE), it was kinetically restricted and could not occur even after 24 h without catalyst (Supporting Information, Figure S7b). By contrast, using Ag NPs@PGMASH composite as catalyst, this reaction occurred quickly, as indicated by a fast color change of the reaction solution from yellow to colorless. The change was quantitatively evaluated using the decreased absorbance at 400 nm and the increased absorbance at 300 nm in UV−vis spectroscopy (Figure 6a). Herein, 4-AP is demonstrated as the sole product of this reaction by the isosbestic points at 252, 280, 318 nm.41,50 The plot of ln(At/A0) against the reaction time was then obtained using the protocol described in section 2.6. Good linear correlation of ln(At/A0) versus time was observed with a correlation coefficient larger than 0.99, confirming the pseudo-first-order rate kinetics (Figure 6b).

XRD patterns of PGMA, PGMA-SH, and Ag NPs@PGMASH composite are shown in Figure 5. Peaks at 2θ angles of 38.3,

Figure 5. XRD patterns of PGMA microspheres (a), PGMA-SH microspheres (b), and Ag NPs@PGMA-SH composite (c).

44.4, 64.9, 77.9, and 81.5 were exhibited, corresponding to the (111), (200), (220), (311), and (222) crystal planes of a cubic lattice structure of Ag NPs, respectively.48 Therefore, the existence of Ag NPs in the crystalline state was confirmed. Moreover, the position and relative intensities were in good agreement with the standard diffraction pattern of Ag (JCPDS 4-783). These results further confirmed that Ag NPs were

Figure 6. Time-dependent UV−vis spectra of the reaction solution in the presence of Ag NPs@PGMA-SH composite (a); plot of ln(At/A0) against the reaction time, corresponding to the reduction of 4-NP (b). 6484

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Industrial & Engineering Chemistry Research Table 2. Recent Studies on the Reduction of 4-NP Using Various Catalysts material Ag NPs@PGMA-SH composite graphene oxide/Ag NPs−Fe3O4 Ag/SiO2 1.08 Ag NPs on nanostructured silica Ag NPs on Fe3O4@C nanocomposites micron-SiO2@Ag NPs

T (°C) RT RT 25 33 25 

diam (nm)

[Ag] (mg·L−1)

16.97 ± 3.15 9−20 19.3 ± 3.8 21 ± 3  10−60

0.9 8.1 1.1 24.1 611.3 107.4

kapp (s−1) −3

(3.94 ± 0.33) × 10 2.67 × 10−2 2.73 × 10−3 1.08 × 10−3 1.70 × 10−2 3.56 × 10−3

k (L·mg−1·s−1)

ref

(4.38 ± 0.37) × 10−3 3.30 × 10−3 2.53 × 10−3 4.48 × 10−5 2.78 × 10−5 3.31 × 10−5

this work 16 53 54 55 2

Figure 7. Conversion of 4-NP in 10 successive cycles with Ag NPs@PGMA-SH composite (a) and TEM image of Ag NPs@PGMA-SH composite after 10 cycles (b).

Scheme 2. Illustration of the Catalytic Reduction of 4-NP with the As-Prepared Ag NPs@PGMA-SH Compositea

a

The colors for various atoms were assigned as yellow for silver, gray for carbon, blue for nitrogen, red for oxygen, and orange for hydrogen.

The kapp was then calculated to be (3.94 ± 0.33) × 10−3 s−1 by the slope, which was much higher than that of other polymersupported catalysts reported elsewhere.51,52 Furthermore, because the kapp was dependent on the amount of Ag NPs in the reaction solution, the “activity parameter” k42 was calculated (see section 2.6), as summarized in Table 2. It can be seen that the k value of Ag NPs in this work was (4.38 ± 0.37) × 10−3 L·mg−1·s−1, which was 1.3−132 times higher than that of Ag NPs loaded on different supports as listed in Table 2.2,16,53−55 It should be noted that the similar k value reported for a graphene oxide/AgNPs−Fe3O4 nanocomposite16 was caused by the special lamellar structure of graphene oxide, which can only be used as a reference here. Recyclability of catalysts is crucial to their practical application. Reuse cycles of the Ag NPs@PGMA-SH composite catalyst were then examined. The catalyst was separated from the reaction solution after each cycle, and then washed and dried for the next run under identical condition (45 mL of 4-

NP solution, 25 °C, 0.5 mg of catalyst, reaction for 15 min). As shown in Figure 7a, a conversion higher than 92% was exhibited even after running of 10 cycles, and no significant change was observed in the size and morphology of the catalyst (Figure 7b). Moreover, no significant aggregation and leakage of Ag NPs was detected even after sonication or washing. A long shelf life of this composite was also confirmed. After storage at room temperature for 6 months, well-dispersed Ag NPs still remained (Supporting Information, Figure S8), and a conversion of 93.8% for the reduction of 4-NP was retained. Therefore, the as-prepared Ag NPs@PGMA-SH composite was proven to have excellent catalytic activity, reusability, and longterm stability, and thus to be a promising catalyst for the borohydride reduction reaction of 4-NP. 3.4. Mechanism of Catalytic Activity Enhancement. On the basis of the above experimental results, a conceivable mechanism for the catalytic reduction of 4-NP by Ag NPs@ PGMA-SH composite was proposed, as described in Scheme 2. 6485

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Industrial & Engineering Chemistry Research The electron transfer from BH4− to Ag NPs and then to 4-NP was the first step, which has been extensively illustrated in the literature with an emphasis on the contribution of the large surface areas of Ag NPs56,57 and electronic properties.16 Herein, BH4− was adsorbed onto the surface of Ag NPs to react and transfer electrons to Ag NPs. The carbons in the epoxy groups of the PGMA-SH microspheres (Scheme 1) were very reactive electrophiles, which could attract electrons and acted as an electron acceptor, resulting in charge distribution between Ag NPs and PGMA-SH microspheres. Meanwhile, N-containing functional groups within the support could introduce a great deal of positively charged active sites13,58 (Scheme 1, Table 1), leading to the preferred capture of the reactant 4-nitrophenolate anions by electrostatic attraction. This could facilitate the accumulation of 4-NP on the polymer surface and then the random collision to the surface of Ag NPs, contributing to the enhancement of electron transfer to 4-NP (Scheme 2). This is an advantage of PGMA-SH support as stated in the literature.32 Thereafter, the reduction of 4-NP to 4-AP was realized, followed by desorption of the product and the regeneration of a free surface for next catalytic cycle. Therefore, the catalytic activity of Ag NPs@PGMA-SH composite is mainly contributed by Ag NPs with fine crystallinity (Figure 5) and large surface area resulted from the nano size (Supporting Information, Figure S6), which is consistent with the mechanism proposed elsewhere.56,57 Moreover, enhanced catalytic activity is obtained, benefiting from the electronic properties of the PGMA-SH microspheres.

S7); TEM image of Ag NPs@PGMA-SH composite stored over 6 months (Figure S8). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b01010.

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Corresponding Author

* Tel.: +86-22-27404981. Fax: +86-22-27403389. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (No. 21236005), the Natural Science Foundation of Tianjin (13JCZDJC31100), the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) (No. 20130032110028), China Scholarship Council (CSC), and the Innovation Foundation of Tianjin University.

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REFERENCES

(1) Sau, T. K.; Rogach, A. L.; Jackel, F.; Klar, T. A.; Feldmann, J. Properties and Applications of Colloidal Nonspherical Noble Metal Nanoparticles. Adv. Mater. 2010, 22, 1805. (2) Wang, M.; Tian, D.; Tian, P.; Yuan, L. Synthesis of MicronSiO2@Nano-Ag Particles and Their Catalytic Performance in 4Nitrophenol Reduction. Appl. Surf. Sci. 2013, 283, 389. (3) Wu, Y.; Wang, D.; Li, Y. Nanocrystals from Solutions: Catalysts. Chem. Soc. Rev. 2014, 43, 2112. (4) Liu, Y.; Goebl, J.; Yin, Y. Templated Synthesis of Nanostructured Materials. Chem. Soc. Rev. 2013, 42, 2610. (5) Liang, M.; Su, R.; Huang, R.; Qi, W.; Yu, Y.; Wang, L.; He, Z. Facile in Situ Synthesis of Silver Nanoparticles on Procyanidin-Grafted Eggshell Membrane and Their Catalytic Properties. ACS Appl. Mater. Interfaces 2014, 6, 4638. (6) Zhang, J.; Zhang, M.; Tang, K.; Verpoort, F.; Sun, T. PolymerBased Stimuli-Responsive Recyclable Catalytic Systems for Organic Synthesis. Small 2014, 10, 32. (7) Kumar, K. S.; Kumar, V. B.; Paik, P. Recent Advancement in Functional Core-Shell Nanoparticles of Polymers: Synthesis, Physical Properties, and Applications in Medical Biotechnology. J. Nanopart. 2013, 1, 1. (8) Zhu, B.; Lu, D.; Ge, J.; Liu, Z. Uniform Polymer−Protein Conjugate by Aqueous AGET ATRP Using Protein as a Macroinitiator. Acta Biomater. 2011, 7, 2131. (9) Gupta, S.; Uhlmann, P.; Agrawal, M.; Chapuis, S.; Oertel, U.; Stamm, M. Immobilization of Silver Nanoparticles on Responsive Polymer Brushes. Macromolecules 2008, 41, 2874. (10) Chen, G. F.; Lu, J. R.; Lam, C.; Yu, Y. A Novel Green Synthesis Approach for Polymer Nanocomposites Decorated with Silver Nanoparticles and Their Antibacterial Activity. Analyst 2014, 139, 5793. (11) Taheri, S.; Baier, G.; Majewski, P.; Barton, M.; Förch, R.; Landfester, K.; Vasilev, K. Synthesis and Surface Immobilization of Antibacterial Hybrid Silver-Poly (L-Lactide) Nanoparticles. Nanotechnology 2014, 25, 305102. (12) Wang, R. Z.; Wang, Z.; Lin, S.; Deng, C.; Li, F.; Chen, Z.; He, H. Green Fabrication of Antibacterial Polymer/Silver Nanoparticle Nanohybrids by Dual-Spinneret Electrospinning. RSC Adv. 2015, DOI: 10.1039/C5RA03288A. (13) Wang, S.; Zhang, J.; Yuan, P.; Sun, Q.; Jia, Y.; Yan, W.; Chen, Z.; Xu, Q. Au Nanoparticle Decorated N-Containing Polymer Spheres: Additive-Free Synthesis and Remarkable Catalytic Behavior for Reduction of 4-Nitrophenol. J. Mater. Sci. 2015, 50, 1323. (14) Mashhadizadeh, M. H.; Karami, Z. Solid Phase Extraction of Trace Amounts of Ag, Cd, Cu, and Zn in Environmental Samples

4. CONCLUSIONS A simple, effective, and straightforward technique for the in situ synthesis of Ag NPs on PGMA-SH microspheres in aqueous solution was proposed. Monodisperse spherical Ag NPs were obtained with a diameter of 16.97 ± 3.15 nm, which showed excellent structural stability due to the stable coordinate covalent bond with sulfhydryl groups of PGMA-SH support. The hybrid Ag NPs@PGMA-SH composite exhibited high catalytic activity in the reduction of 4-NP, as a result of the high surface area and fine crystallinity of Ag NPs and electronic properties of PGMA-SH supports. Excellent reproducibility and stability were further confirmed. Moreover, the nucleation and growth of Ag NPs on the flexible polymer substrate suggested that this approach could be applied to other metal−polymeric substrate materials. Following this approach, the promising hybrid composites would realize the conversion of nitro to amino compounds on a large scale and would have potential applications in environmental control and biomedicines.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

Ellman test of the resuspension solution of PGMA and PGMASH microspheres (Figure S1); photographs of PGMA-SH microspheres and Ag NPs@PGMA-SH composite (Figure S2); UV−vis spectrum of the supernatant after the formation of Ag NPs on PGMA-SH microspheres (Figure S3); TEM images of Ag NPs@PGMA-SH composites prepared with different amounts of AgNO3 (Figure S4); TEM image of microtomed Ag NPs@PGMA-SH composite (Figure S5); size distribution histogram of Ag NPs on PGMA-SH microspheres (Figure S6); UV−vis spectra of 4-NP before and after the addition of NaBH4 and UV−vis spectra of 4-NP and NaBH4 mixed aqueous solution freshly prepared or 24 h after preparation in the absence of Ag NPs@PGMA-SH aqueous dispersion (Figure 6486

DOI: 10.1021/acs.iecr.5b01010 Ind. Eng. Chem. Res. 2015, 54, 6480−6488

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Industrial & Engineering Chemistry Research

(33) Ran, X.; Sun, H.; Pu, F.; Ren, J.; Qu, X. Ag NanoparticleDecorated Graphene Quantum Dots for Label-Free, Rapid and Sensitive Detection of Ag+ and Biothiols. Chem. Commun. 2013, 49, 1079. (34) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 1103. (35) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. Comparison of the Structures and Wetting Properties of Self-Assembled Monolayers of N-Alkanethiols on the Coinage Metal Surfaces, Copper, Silver, and Gold. J. Am. Chem. Soc. 1991, 113, 7152. (36) Sun, S.-K.; Wang, H.-F.; Yan, X.-P. A Sensitive and Selective Resonance Light Scattering Bioassay for Homocysteine in Biological Fluids Based on Target-Involved Assembly of PolyethyleneimineCapped Ag-Nanoclusters. Chem. Commun. 2011, 47, 3817. (37) Giersig, M.; Mulvaney, P. Preparation of Ordered Colloid Monolayers by Electrophoretic Deposition. Langmuir 1993, 9, 3408. (38) Supriya, L.; Claus, R. O. Solution-Based Assembly of Conductive Gold Film on Flexible Polymer Substrates. Langmuir 2004, 20, 8870. (39) Shi, S.; Zhang, L.; Wang, T.; Wang, Q.; Gao, Y.; Wang, N. Poly(N-Isopropylacrylamide)−Au Hybrid Microgels: Synthesis, Characterization, Thermally Tunable Optical and Catalytic Properties. Soft Matter 2013, 9, 10966. (40) Horák, D.; Shapoval, P. Reactive Poly (Glycidyl Methacrylate) Microspheres Prepared by Dispersion Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3855. (41) Shin, H.-S.; Huh, S. Au/Au@Polythiophene Core/Shell Nanospheres for Heterogeneous Catalysis of Nitroarenes. ACS Appl. Mater. Interfaces 2012, 4, 6324. (42) Lara, L. R.; Zottis, A. D.; Elias, W. C.; Faggion, D.; de Campos, C. E. M.; Acuña, J. J. S.; Domingos, J. B. The Catalytic Evaluation of in Situ Grown Pd Nanoparticles on the Surface of Fe3O4@Dextran Particles in the P-Nitrophenol Reduction Reaction. RSC Adv. 2015, 5, 8289. (43) Cheng, J.; Wang, Y.; Teng, C.; Shang, Y.; Ren, L.; Jiang, B. Preparation and Characterization of Monodisperse, Micrometer-Sized, Hierarchically Porous Carbon Spheres as Catalyst Support. Chem. Eng. J. 2014, 242, 285. (44) Thielbeer, F.; Donaldson, K.; Bradley, M. Zeta Potential Mediated Reaction Monitoring on Nano and Microparticles. Bioconjugate Chem. 2011, 22, 144. (45) Liang, M.; Wang, L.; Su, R.; Qi, W.; Wang, M.; Yu, Y.; He, Z. Synthesis of Silver Nanoparticles within Cross-Linked Lysozyme Crystals as Recyclable Catalysts for 4-Nitrophenol Reduction. Catal. Sci. Technol. 2013, 3, 1910. (46) Kuroda, K.; Ishida, T.; Haruta, M. Reduction of 4-Nitrophenol to 4-Aminophenol over Au Nanoparticles Deposited on PMMA. J. Mol. Catal. A: Chem. 2009, 298, 7. (47) Jiang, B.; Tian, C.; Song, G.; Chang, W.; Wang, G.; Wu, Q.; Fu, H. A Novel Ag/Graphene Composite: Facile Fabrication and Enhanced Antibacterial Properties. J. Mater. Sci. 2012, 48, 1980. (48) Ghaedi, M.; Biyareh, M. N.; Kokhdan, S. N.; Shamsaldini, S.; Sahraei, R.; Daneshfar, A.; Shahriyar, S. Comparison of the Efficiency of Palladium and Silver Nanoparticles Loaded on Activated Carbon and Zinc Oxide Nanorods Loaded on Activated Carbon as New Adsorbents for Removal of Congo Red from Aqueous Solution: Kinetic and Isotherm Study. Mater. Sci. Eng., C 2012, 32, 725. (49) Dauthal, P.; Mukhopadhyay, M. Prunus Domestica Fruit Extract-Mediated Synthesis of Gold Nanoparticles and Its Catalytic Activity for 4-Nitrophenol Reduction. Ind. Eng. Chem. Res. 2012, 51, 13014. (50) Saha, S.; Pal, A.; Kundu, S.; Basu, S.; Pal, T. Photochemical Green Synthesis of Calcium-Alginate-Stabilized Ag and Au Nanoparticles and Their Catalytic Application to 4-Nitrophenol Reduction. Langmuir 2010, 26, 2885.

Using Magnetic Nanoparticles Coated by 3-(Trimethoxysilyl)-1Propantiol and Modified with 2-Amino-5-Mercapto-1,3,4- Thiadiazole and Their Determination by ICP-OES. J. Hazard. Mater. 2011, 190, 1023. (15) Ai, L.; Zeng, C.; Wang, Q. One-Step Solvothermal Synthesis of Ag-Fe3O4 Composite as a Magnetically Recyclable Catalyst for Reduction of Rhodamine B. Catal. Commun. 2011, 14, 68. (16) Qu, J.-C.; Ren, C.-L.; Dong, Y.-L.; Chang, Y.-P.; Zhou, M.; Chen, X.-G. Facile Synthesis of Multifunctional Graphene Oxide/ AgNPs-Fe3O4 Nanocomposite: A Highly Integrated Catalysts. Chem. Eng. J. 2012, 211−212, 412. (17) Huang, Q.; Wang, J.; Wei, W.; Yan, Q.; Wu, C.; Zhu, X. A Facile and Green Method for Synthesis of Reduced Graphene Oxide/Ag Hybrids as Efficient Surface Enhanced Raman Scattering Platforms. J. Hazard. Mater. 2015, 283, 123. (18) Song, Y.; Cui, K.; Wang, L.; Chen, S. The Electrodeposition of Ag Nanoparticles on a Type I Collagen-Modified Glassy Carbon Electrode and Their Applications as a Hydrogen Peroxide Sensor. Nanotechnology 2009, 20, 105501. (19) Wang, D. H.; Park, K. H.; Seo, J. H.; Seifter, J.; Jeon, J. H.; Kim, J. K.; Park, J. H.; Park, O. O.; Heeger, A. J. Enhanced Power Conversion Efficiency in PCDTBT/PC70BM Bulk Heterojunction Photovoltaic Devices with Embedded Silver Nanoparticle Clusters. Adv. Energy Mater. 2011, 1, 766. (20) Kondo, T.; Lee, S. M.; Malicki, M.; Domercq, B.; Marder, S. R.; Kippelen, B. A Nonvolatile Organic Memory Device Using ITO Surfaces Modified by Ag-Nanodots. Adv. Funct. Mater. 2008, 18, 1112. (21) Nguyen, V. H.; Shim, J.-J. Facile Synthesis and Characterization of Carbon Nanotubes/Silver Nanohybrids Coated with Polyaniline. Synth. Met. 2011, 161, 2078. (22) Zhu, J.; Zhang, Y.; Lu, D.; Zare, R. N.; Ge, J.; Liu, Z. Temperature-Responsive Enzyme−Polymer Nanoconjugates with Enhanced Catalytic Activities in Organic Media. Chem. Commun. 2013, 49, 6090. (23) Zhao, C.; Li, X.; Li, L.; Cheng, G.; Gong, X.; Zheng, J. Dual Functionality of Antimicrobial and Antifouling of Poly (N-Hydroxyethylacrylamide)/Salicylate Hydrogels. Langmuir 2013, 29, 1517. (24) Zhu, Z.; Guo, X.; Wu, S.; Zhang, R.; Wang, J.; Li, L. Preparation of Nickel Nanoparticles in Spherical Polyelectrolyte Brush Nanoreactor and Their Catalytic Activity. Ind. Eng. Chem. Res. 2011, 50, 13848. (25) Xu, L. Q.; Yap, B. S. M.; Wang, R.; Neoh, K.-G.; Kang, E.-T.; Fu, G. D. Catecholamine-Induced Electroless Metallization of Silver on Silica@Polymer Hybrid Nanospheres and Their Catalytic Applications. Ind. Eng. Chem. Res. 2014, 53, 3116. (26) Zhu, H.; Ma, Z.; Clark, J. C.; Pan, Z.; Overbury, S. H.; Dai, S. Low-Temperature Co Oxidation on Au/Fumed SiO2-Based Catalysts Prepared from Au(en)2Cl3 Precursor. Appl. Catal., A 2007, 326, 89. (27) Lam, E.; Hrapovic, S.; Majid, E.; Chong, J. H.; Luong, J. H. Catalysis Using Gold Nanoparticles Decorated on Nanocrystalline Cellulose. Nanoscale 2012, 4, 997. (28) Oh, J. S.; Dang, L. N.; Yoon, S. W.; Nam, J. D. AmineFunctionalized Polyglycidyl Methacrylate Microsphere as a Unified Template for the Synthesis of Gold Nanoparticles and Single-Crystal Gold Plates. Macromol. Rapid Commun. 2013, 34, 504. (29) Liu, Y.; Li, M.; Chen, G. A New Type of Raspberry-Like Polymer Composite Sub-Microspheres with Tunable Gold Nanoparticles Coverage and Their Enhanced Catalytic Properties. J. Mater. Chem. A 2013, 1, 930. (30) Takahashi, K.; Uyama, H.; Kobayashi, S. Preparation of Reactive Monodisperse Particles in the Micron Range by Dispersion Polymerization of Glycidyl Methacrylate. Polym. J. 1998, 30, 684. (31) Song, T.; Zhou, M.; Liu, W.; Bian, G.; Qi, Y.; Bai, F.; Yang, X. Preparation of Polymer Microspheres with Reactive Epoxy Group and Amino Groups as Stabilizers for Gold Nanocolloids with Recoverable Catalysis. Colloid Polym. Sci. 2014, 293, 187. (32) Li, M.; Chen, G. Revisiting Catalytic Model Reaction PNitrophenol/NaBH4 Using Metallic Nanoparticles Coated on Polymeric Spheres. Nanoscale 2013, 5, 11919. 6487

DOI: 10.1021/acs.iecr.5b01010 Ind. Eng. Chem. Res. 2015, 54, 6480−6488

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Industrial & Engineering Chemistry Research (51) Huang, X.; Xiao, Y.; Zhang, W.; Lang, M. In-Situ Formation of Silver Nanoparticles Stabilized by Amphiphilic Star-Shaped Copolymer and Their Catalytic Application. Appl. Surf. Sci. 2012, 258, 2655. (52) Butun, S.; Sahiner, N. A Versatile Hydrogel Template for Metal Nano Particle Preparation and Their Use in Catalysis. Polymer 2011, 52, 4834. (53) Horecha, M.; Kaul, E.; Horechyy, A.; Stamm, M. Polymer Microcapsules Loaded with Ag Nanocatalyst as Active Microreactors. J. Mater. Chem. A 2014, 2, 7431. (54) Das, S. K.; Khan, M. M. R.; Guha, A. K.; Naskar, N. Bio-Inspired Fabrication of Silver Nanoparticles on Nanostructured Silica: Characterization and Application as a Highly Efficient Hydrogenation Catalyst. Green Chem. 2013, 15, 2548. (55) Zhu, M.; Wang, C.; Meng, D.; Diao, G. In Situ Synthesis of Silver Nanostructures on Magnetic Fe3O4@C Core−Shell Nanocomposites and Their Application in Catalytic Reduction Reactions. J. Mater. Chem. A 2013, 1, 2118. (56) Jiang, Y.; Li, G.; Li, X.; Lu, S.; Wang, L.; Zhang, X. WaterDispersible Fe3O4 Nanowires as Efficient Supports for Noble-Metal Catalysed Aqueous Reactions. J. Mater. Chem. A 2014, 2, 4779. (57) Zhang, P.; Shao, C.; Zhang, Z. In Situ Assembly of WellDispersed Ag nanoparticles (AgNPs) on Electrospun Carbon Nanofibers (CNFs) for Catalytic Reduction of 4-Nitrophenol. Nanoscale 2011, 3, 3357. (58) Kong, X.-K.; Sun, Z.-Y.; Chen, M.; Chen, Q.-W. Metal-Free Catalytic Reduction of 4-Nitrophenol to 4-Aminophenol by N-Doped Graphene. Energy Environ. Sci. 2013, 6, 3260.

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DOI: 10.1021/acs.iecr.5b01010 Ind. Eng. Chem. Res. 2015, 54, 6480−6488